Transistor-based filter for inhibiting load noise from entering a power supply

ABSTRACT

A transistor-based filter for inhibiting load noise from entering a power supply is disclosed. The filter includes a first transistor having an emitter coupled to a power supply, a collector coupled to a load, and a base. The filter also includes a first capacitor coupled between the base of the first transistor and a ground terminal The filter further includes an impedance coupled between the base and a node between the collector and the load, or a second transistor and second capacitor. The impedance can be a resistor or an inductor.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from and is a continuation of U.S.patent application Ser. No. 13/397,928 filed by the same inventor onFeb. 16, 2012 and entitled “Transistor Based Filter For Inhibiting LoadNoise From Entering A Power Supply,” the contents of which are herebyincorporated by reference

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The invention was made with Government support under ContractDE-AC05-76RLO1830, awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

TECHNICAL FIELD

This invention relates to filters. More specifically, this inventionrelates to a transistor-based filter that inhibits load noise fromentering a power supply.

BACKGROUND OF THE INVENTION

Filtering electrical power can be an important aspect of powerdistribution systems at many levels, from commercial AC powerdistribution down to the DC power rails within an instrument thatperforms sensitive tasks. There are many sources of electrical noisefound in modern electronics, including line noise, RF noise from highfrequency circuits, and digital noise from computer subsystems. Problemsarise when noise-sensitive components must run on power systems alreadypolluted by noisy sub-systems. While inductor-capacitor filters areeffective both for sheltering sensitive components and isolating noisyones in many applications, they are not practical for high levels ofbroadband low frequency noise, such as is typical of modern DC electriccooling fans.

Filters can take a variety of forms, some of the simplest being the RCand LC filters shown in FIGS. 1A and 1B respectively. Passive filtersare extremely common, and are used in both signal and power circuitry.For example, the low-pass filters shown in the FIG. 1A and FIG. 1B couldbe used to attenuate AC components present on a DC power rail, or toseparate low and high frequency AC signals. In many cases, the action ofpassive networks is reversible, meaning that they have a similar filterbehavior if turned end-for-end. Consequently, such networks, if used toremove noise, can do so in both directions, thus protecting the loadfrom the source, and the source from the load.

Well-designed LC filters are superb both as signal extraction or noiserejection circuits for frequencies from the tens of kilohertz into themegahertz. However, below a kilohertz, components for LC filters andcapacitors for RC filters become progressively larger as frequencyfurther decreases. The broadband performance of such low frequencyfilter components can also be sub-optimal, requiring multiple stages offiltering for different frequency ranges. Active filters such as thatshown in FIG. 2 answer some of these problems for signal filter systems,allowing physically small circuits to have corner frequencies of a Hertzor less, while retaining correct behavior into the hundreds of kilohertzor low megahertz in many cases. However, active filters such as this donot pass power from the input to the output, so they are not powerfiltering devices.

In order to filter low frequency noise from DC power rails in an activemanner, transistor-based filters are effective, although far lessstudied than operational amplifier-based circuits. A basic transistorfilter is shown in FIG. 3. This circuit is also known as a capacitancemultiplier. The transistor Q passes current to any load attached toV_(out) via its emitter, a very low impedance connection. Since thebase-emitter voltage (V_(be)) of a bipolar junction transistor (BJT)varies as the logarithm of the collector current, V_(out) varies onlyslightly for significant changes in load current. (At a room temperatureof 300 Kelvin, a 10-fold increase in collector current corresponds to anincrease in V_(be) of approximately 60 mV.) In contrast, current isdelivered to this circuit via the collector of Q, a high impedanceterminal Provided it remains above the saturation value of Q for a givencurrent flow, the collector-base voltage (V_(cb)) of the transistor canvary over the full range specified for the device, with little impact onV_(be). (There is a small coupling between the two through the Earlyeffect, but this is negligible in this circuit.) The importantconsequence is that the voltage appearing at the emitter of Q, isprimarily determined by the voltage at its base, which in this circuitequals the voltage appearing across capacitor C. This voltage is in turna filtered version of input V_(in), fluctuations in voltage above thecorner frequency of the RC network being attenuated. Finally, because ofthe near constant V_(be) value of Q, fluctuations in V_(in) above the RCcorner frequency are also attenuated at V_(out),

Transistor Q acts as a voltage follower, or a current amplifier in thisfilter circuit. While the voltage characteristics appearing at V_(out)are very similar to those at the base of Q, the current capability atthese two nodes is very different. The current passing out the emitterof a typical BJT (when operating in its linear region) are typicallyhundreds of times that into its base. Because of this, the currentcapacity of R and C can be small (i.e., large R-value and smallC-value), and thus their physical sizes can be small for a given RC timeconstant or filtering capability. Since the voltage drop across Q(V_(ce)) is in general around a volt for this circuit, Q does notdissipate large amounts of power, and can thus also be small despite alarge current capacity. Consequently, these three components can make avery effective low-pass filter for low frequency noise, with muchsmaller components than the equivalent LC or RC power filter that passesthe same current flow.

While the circuit of FIG. 3 protects the load from noise in a powersupply, the reverse is not true. The power supply is not protected fromnoise developed in the load. Load-induced voltage changes are caused byrapid changes in current demand. Such current spikes or surges wouldpropagate directly back through transistor Q into the power supplythrough V_(in). This in turn would cause a voltage change across thepower supply commensurate with its output impedance, in a similar manneras would occur if the filter were omitted from the circuit.

What is needed is a transistor-based filter that protects the powersupply from noise developed in the load.

SUMMARY OF THE INVENTION

The present invention is directed to a transistor-based filter forinhibiting load noise from entering a power supply. In one embodiment,the filter includes a first transistor with an emitter coupled to apower supply, a collector coupled to a load, and a base. The filter alsoincludes a first capacitor coupled between the base and a groundterminal and a first resistor coupled between the base and a nodebetween the collector and the load. The first capacitor charges throughthe base of the first transistor, turning the first transistor on. Thefirst resistor pulls a bias current from the base of the firsttransistor to keep it turned on after the first capacitor has charged.

During normal operation, the first transistor is insensitive to noisevoltages developed across the load. Voltage noise generated in the loadis inhibited from coupling upstream to the power supply through thefirst transistor.

In one embodiment, the transistor-based filter further comprises atleast one diode coupled between the base, and the collector of thetransistor, wherein an anode of the at least one diode is coupled to thebase and a cathode of the at least one diode is coupled to thecollector.

In another embodiment, the transistor-based filter further comprises atleast one zener diode inserted between the base and the collector of thetransistor, wherein a cathode of the at least one zener diode is coupledto the base, and an anode of the at least one zener diode is coupled tothe collector. In another embodiment, the at least one zener diode is inseries with at least one diode in either order, provided polarity withrespect to the transistor is preserved.

In another embodiment, the transistor-based filter also comprises asecond resistor inserted between the first resistor and the node betweenthe collector and the load; and a second capacitor coupled from the nodebetween the first and the second resistors to the ground terminal.

In another embodiment, the transistor-based filter also comprises athird capacitor coupled between the node common to the collector and theload, and the ground terminal.

In one embodiment, the transistor-based filter also comprises a firstinductor coupled between the power supply and the emitter.

In another embodiment, the transistor-based filter also comprises afourth capacitor coupled between a node common to the power supply andthe first inductor, and the ground terminal.

In another embodiment, the transistor-based filter also comprises athird resistor inserted between the base of the first transistor and thefirst capacitor.

In another embodiment, the transistor-based filter also comprises atleast one diode connected from the emitter of the first transistor tothe first capacitor, wherein an anode of the at least one diode isconnected to the emitter of the first transistor and a cathode of the atleast one diode is connected to the non-grounded end of the firstcapacitor.

In another embodiment, the transistor-based filter also comprises asnubber network to reduce oscillations, if any, occurring at the nodecommon to the first inductor and the first transistor. In oneembodiment, the snubber network is coupled from the node between thefirst inductor and the first transistor to the ground terminal. Inanother embodiment, the snubber network is coupled from the node betweenthe first inductor and the first transistor to the base of the firsttransistor. In another embodiment, the snubber network is coupled fromthe node between the first inductor and the first transistor to thepower supply.

The load can be, but is not limited to, a fan or a pump. In oneembodiment, the fan is a brushless DC fan.

The load can be a device drawing a periodically varying current, such asa current-modulated or current-swept laser. The laser can be a quantumcascade laser.

Preferably, the transistor-based filter of the present invention is usedin a system in which multiple loads are connected to the same powersupply, either directly or indirectly.

In one embodiment, the transistor-based filter includes a secondtransistor. A base of the second transistor is coupled to the emitter ofthe first transistor; a fourth resistor is inserted between the emitterof the first transistor and the first inductor; a collector of thesecond transistor is coupled to the collector of the first transistor;an emitter of the second transistor is coupled to the node between thefirst inductor and the fourth resistor.

The transistor-based filter can further include a third transistor. Abase of the third transistor is coupled to the collector of the firsttransistor; a fifth resistor is inserted between the collector of thefirst transistor and the load; a collector of the third transistor iscoupled to the emitter of the first transistor; an emitter of the thirdtransistor is coupled to the node between the fifth resistor and theload.

In another embodiment a fifth capacitor is coupled between the base andcollector of the first transistor.

In another embodiment of the present invention, a transistor-basedfilter for inhibiting load noise from entering a power supply isdisclosed. The filter includes a first transistor with an emittercoupled to a power supply, a collector coupled to a load, and a base.The filter also includes a first capacitor coupled between the base anda ground terminal. The first capacitor charges through the base of thefirst transistor, turning the first transistor on. The filter furtherincludes a first inductor coupled between the base and a node betweenthe collector and the load, wherein the first inductor pulls a biascurrent from the base of the first transistor to keep it turned on afterthe first capacitor has charged.

In one embodiment, the transistor-based filter further comprises atleast one diode coupled between the base and the collector of thetransistor.

In one embodiment, the transistor-based filter further comprises a firstresistor inserted between the first inductor and the node between thecollector and the load; and a second capacitor coupled from the nodebetween the first inductor and the first resistor to the groundterminal.

In another embodiment, the transistor-based filter also comprises asecond inductor coupled between the power supply and the emitter.

In another embodiment, the transistor-based filter also comprises athird capacitor coupled between the node common to the collector and theload, and the ground terminal.

In another embodiment, the transistor-based filter also comprises afourth capacitor coupled between a node common to the power supply andthe second transistor, and the ground terminal.

In another embodiment, the transistor-based filter also comprises asecond resistor inserted between the base of the first transistor andthe first capacitor.

In another embodiment, the transistor-based filter comprises a snubbernetwork to reduce oscillations, if any, occurring at the node common tothe second inductor and the first transistor. In one embodiment, thesnubber network is coupled from the node between the second inductor andthe first transistor to the ground terminal. In another embodiment, thesnubber network is coupled from the node between the second inductor andthe first transistor to the base of the first transistor. In anotherembodiment, the snubber network is coupled from the node between thesecond inductor and the first transistor to the power supply.

In one embodiment, the transistor-based filter includes a secondtransistor. A base of the second transistor is coupled to the emitter ofthe first transistor; a third resistor is inserted between the emitterof the first transistor and the second inductor; a collector of thesecond transistor is coupled to the collector of the first transistor;an emitter of the second transistor is coupled to the node between thesecond inductor and the third resistor.

The transistor-based filter can further include a third transistor. Abase of the third transistor is coupled to the collector of the firsttransistor; a fourth resistor is inserted between the collector of thefirst transistor and the load; a collector of the third transistor iscoupled to the emitter of the first transistor; an emitter of the thirdtransistor is coupled to the node between the fourth resistor and theload.

In another embodiment of the present invention, a transistor-basedfilter for inhibiting load noise from entering a power supply isdisclosed. The filter includes a first transistor with an emittercoupled to a power supply, a collector coupled to a load, and a base.The filter also includes a first capacitor coupled between the base ofthe first transistor and a ground terminal. The first capacitor chargesthrough the base of the first transistor, turning the first transistoron. The filter further includes a second transistor with an emittercoupled to the node between the base of the first transistor and thefirst capacitor. The filter also includes a first resistor coupledbetween a collector of the second transistor and the load. The filteralso includes a second capacitor coupled between a base of the secondtransistor and the ground terminal. The filter also includes a secondresistor coupled between the base and collector of the secondtransistor, to keep the second transistor biased on during normaloperation.

In one embodiment, a third resistor is connected between the emitter andthe base of the first transistor.

In another embodiment, a fourth resistor is inserted between the base ofthe first transistor and the node between the first capacitor and theemitter of the second transistor.

In another embodiment, a diode network is connected between the nodecommon to the first inductor and the emitter of the first transistor,and both the first and second capacitors, wherein the diode networkcharges the first and second capacitors rapidly upon startup, and limitsthe voltage developed across the fourth resistor, thus limiting thecurrent passing through the base-emitter junctions of both the first andsecond transistors.

In another embodiment, the transistor-based filter further comprises atleast one diode coupled between the base and the collector of the secondtransistor, wherein an anode of the at least one diode is coupled to thebase, and a cathode of the at least one diode is coupled to thecollector.

In another embodiment, the transistor-based filter also comprises athird capacitor coupled between the node common to the collector of thefirst transistor and the load, and the ground terminal.

In another embodiment, the transistor-based filter also comprises afirst inductor coupled between the power supply and the emitter of thefirst transistor.

In another embodiment, the transistor-based filter also comprises afourth capacitor coupled between a node common to the power supply andthe first inductor, and the ground terminal.

In another embodiment, the transistor-based filter comprises a snubbernetwork to reduce oscillations, if any, occurring at the node common tothe first inductor and the first transistor. In one embodiment, thesnubber network is coupled from the node between the first inductor andthe first transistor to the ground terminal. In another embodiment, thesnubber network is coupled from the node between the first inductor andthe emitter of the first transistor to the base of the first transistor.In another embodiment, the snubber network is coupled from the nodebetween the first inductor and the first transistor to the power supply.

In one embodiment, the transistor-based filter comprises a thirdtransistor. A base of the third transistor is coupled to the emitter ofthe first transistor; a fifth resistor is inserted between the emitterof the first transistor and the first inductor; a collector of the thirdtransistor is coupled to the collector of the first transistor; anemitter of the third transistor is coupled to the node between the firstinductor and the fifth resistor.

In another embodiment, the transistor-based filter also comprises afourth transistor, wherein a base of the fourth transistor is coupled toa node common to the load and the collector of the first transistor; asixth resistor is inserted between the collector of the first transistorand the load; a collector of the fourth transistor is coupled to theemitter of the first transistor; an emitter of the fourth transistor iscoupled to the node between the sixth resistor and the load.

In another embodiment a fifth capacitor is coupled between the base andcollector of the first transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate basic RC and LC passive filters.

FIG. 2 illustrates a basic active filter.

FIG. 3 illustrates a basic transistor-based filter known as acapacitance multiplier.

FIG. 4 illustrates a transistor-based filter for inhibiting load noisefrom entering a power supply, in accordance with one embodiment of thepresent invention.

FIG. 5 illustrates a transistor-based filter for inhibiting load noisefrom entering a power supply, in accordance with another embodiment ofthe present invention.

FIG. 6 illustrates a transistor-based filter for inhibiting load noisefrom entering a power supply, in accordance with another embodiment ofthe present invention.

FIG. 7 illustrates a transistor-based filter for inhibiting load noisefrom entering a power supply, in accordance with another embodiment ofthe present invention.

FIG. 8 shows power supply voltage fluctuations versus time, induced by abrushless DC cooling fan, for circuits using certain embodiments of thepresent invention.

FIG. 9 shows power supply voltage fluctuations versus time, induced by abrushless DC cooling fan, for circuits using certain embodiments of thepresent invention.

FIG. 10 shows power supply voltage spectral density, resulting from abrushless DC cooling fan, for circuits using certain embodiments of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is directed to a transistor-based filter thatallows high currents to a load if necessary and filters noise from thatload from getting back into the power supply. Existing transistorfilters operate by preventing power supply noise from entering the load,rather than protecting a power distribution system from noisy loads suchas brushless DC fan motors. In the present invention, the filteringaction is opposite to the power flow.

FIG. 4 illustrates a transistor-based filter 100 for inhibiting loadnoise from entering a power supply, in accordance with one embodiment ofthe present invention. The filter 100 comprises a transistor 130 with anemitter 131 arranged to receive a positive connection of an inputvoltage V_(in), a collector 132 arranged to provide an output voltageV_(out) to a load, and a base 133 coupled to a junction of a capacitor150 connected to ground 160 and a resistor 140 coupled to a node betweenthe collector 132 and V_(out).

Compared to the circuit of FIG. 3, the circuit of FIG. 4 uses theopposite polarity of the transistor, and the location of the resistorhas changed. Electric current and power still flow through the circuitof FIG. 4 from left to right, but the high and low impedance terminalsof the transistor have been swapped. Hence, in FIG. 4, the input voltageV_(in) is relatively unaffected by the output voltage V_(out) because ofthe high impedance of the collector of the transistor. In essence, thedirection of the filtering action has been reversed. The power supply isprotected from noise developed in the load.

Also, unlike the circuit of FIG. 3, a current surge through thetransistor to the power supply cannot occur in the circuit of FIG. 4.This is because in FIG. 4, the load sees the high impedance collector ofthe transistor, which acts as a current source and naturally regulatescurrent flow. Any attempt of the load to pull a current surge from thefilter results in a drop in V_(out). Rather than producing a large surgecurrent through the circuit, it causes a linear increase in base currentthrough the transistor because of the increased voltage drop acrossresistor 140, which in turn results in a linear increase in load currentthrough the amplifying action of the transistor, and thus a linearincrease in current demand from the power supply. Assuming low impedanceof the power supply driving the filter, such a linear increase incurrent would cause a negligible change in power rail voltage.(Conversely, a relatively small change in input voltage applied to thefilter can suddenly increase V_(out) or drop it momentarily to zero. Inother words this particular filter configuration does not shield theload from power supply fluctuations.)

FIG. 5 illustrates a transistor-based filter 200 for inhibiting loadnoise from entering a power supply, in accordance with anotherembodiment of the present invention. The RC filter is replaced with atwo-stage filter, significantly improving the noise rejection of thecircuit. An output capacitor 230 is added immediately before the load220, providing a low impedance pathway for rapid signals to the load. Itacts as a storage mechanism for the surge current, which a noisy loadsuch as a brushless DC fan motor may demand. More importantly, thecirculation of this current can be kept localized to the outputcapacitor 230 and the load 220, keeping the surge current from passingthrough the ground plane. A diode 250 and zener diode 251 are added,which prevent the collector 242 of the transistor 240 from falling morethan a predetermined amount below the base 243 by pulling more basecurrent, thus turning on the transistor 240 harder. The values of filterresistors 271 and 272 need to be calculated in order to providesufficient base current, given this predetermined maximum voltage drop,corresponding to a given load current and a given transistor. Otherwisediodes 250 and 251 will conduct all the time and the filter 200 will beineffective. Any series combination of rectifier diodes, zener diodesand Schottky diodes can be used.

The inductor 260 added between the power supply 210 and the emitter 241of the transistor 240 enhances the action of the filter 200. Since thetransistor 240 in this configuration is very sensitive to voltage changeat its emitter 241, any counter electromotive force (EMF) generated bythe inductor 260 in response to a changing current through its windings,produces a larger impact on that current flow than would result from theaction of inductor 260 alone. However, since this configuration can tendto cause oscillations due to time delays between the counter EMFgenerated by the inductor 260 and the action of the transistor 240,snubber components may be required. These can include the resistor 295across the inductor 260, the resistor 290 from the emitter 241 to thebase 243, and the resistor-capacitor combination 282 and 281 from thenode between the inductor 260 and the emitter 241, to ground 215. Thefilter 200 of FIG. 5 also includes filter resistors (or filterimpedances) 271 and 272, filter capacitors 285 and 287, and a line 275designating other components coupled to the circuit.

The resistor 275 inserted between the base 243 of the transistor 240 andthe capacitor 285, works in concert with the diodes 255 and 257connected between the emitter 241 of the transistor 240, and the nodebetween the resistor 275 and the capacitor 285, to limit the currentthrough the base-emitter junction of the transistor 240 upon initialapplication of the power to the filter circuit 200.

In another embodiment of the present invention, as illustrated in FIG.6, a transistor-based filter 300 for inhibiting load noise from enteringa power supply is disclosed. Filter 300 is similar to Filter 200, exceptthat it comprises a complementary Darlington transistor pair, 330 and340. Filter 300 further includes reverse polarity input protection diode310 and transistor protection diode 320.

The input protection diode 310 protects the filter 300 (and connectedload) from a reverse voltage connection to the input, V_(in). Thecomplementary Darlington transistor pair 330 and 340, provide largeroutput currents than a single transistor configuration. The transistorprotection diode 320 prevents damage to the transistor pair 330 and 340due to reverse current flow in the event of the shorting of inputV_(in).

In another embodiment of the present invention, as illustrated in FIG.7, a transistor-based filter 400 for inhibiting load noise from enteringa power supply is disclosed. Filter 400 is similar to Filter 300, exceptthat it comprises two transistor-based filters, one nested within theother. Complementary Darlington transistor pair 430 and 440 andsurrounding components constitute a transistor-based filter similar toFilter 300. Transistor 450, capacitor 460 and resistor 470 constitute asecond transistor-based filter inserted in the current pathway for thebias of Darlington pair 430 and 440 of the first transistor filter. Inthis manner, the bias current supplied to Darlington pair 430 and 440 isfiltered by the second transistor filter comprising transistor 450,capacitor 460 and resistor 470. The diode network 402, 404, 406 and 408and the resistor 410 protect the base-emitter junctions of transistors430 and 450 upon application of power to the filter at V_(in).

In another embodiment, the diode network 402, 404, 406 and 408, can bereplaced with other components that have a similar functionality todiodes, or protect transistors 430 and 450 in a similar manner.

Experimental Section

The following examples serve to illustrate certain exemplary embodimentsand aspects of the present invention and are not to be construed aslimiting the scope thereof.

Experimental Set-up and Results

Comparative measurements were taken using several different embodimentsof reverse transistor filter (herein also referred to as RQF), withsimilar time constants and voltage drops. A commercial 24-V brushless DCfan was used as the load, drawing approximately 70 mA average currentfrom the power supply. A 24-V DC battery bank was used as the powersupply, which provided an electrically quiet background for themeasurements being performed. Measurements of the noise at the powersupply and the fans were taken, both versus time, and in spectraldensity format versus frequency, in order to compare the performance ofvarious filter configurations.

One embodiment tested was similar to embodiment 200 shown in FIG. 5,wherein transistor 240 was a 2N2907A. The following components wereabsent: resistors 282, 295, leaving their connections open; resistor272, leaving its connections shorted; capacitors 281 and 287, leavingtheir connections open; diodes 250 and 251, leaving their connectionsopen. Capacitors 230, 285 and 270 were 100 μF; resistor 271 was 10 kΩ;resistor 290 was 56 kΩ; resistor 275 was 100Ω; inductor 260 was 470 μH;diodes 255 and 257 were 1N4006. This embodiment is referred to in thespectral density plot shown in FIG. 10 as RQF; the inductor 260 could beshorted out or left intact, these two variations being referenced by RQFNO INDUCTOR and RQF WITH INDUCTOR in the time dependent plots shown inFIGS. 8 and 9.

Another embodiment tested was similar to embodiment 400 shown in FIG. 7,wherein transistors 430 and 450 were 2N3906, and transistor 440 was a2N2222A. Resistor 470 was 1 MΩ; capacitor 460 was 10 μF. The remainingcomponents were similar to those described above for RQF. Thisembodiment is referred to in both spectral density and time dependentplots as NESTED RQF.

Another circuit tested was similar to that shown in FIG. 1A, where theresistor was 27 Ω, and the capacitor was 200 μF. This embodiment wasreferred to as RC FILTER.

A remaining test performed was a measurement of the power supply voltagecharacteristics with the fan connected to the power supply directly,wherein no filter was inserted between the fan and the power supply,this arrangement being referred to as FAN, NO FILTER.

A remaining test performed was a measurement of the power supply voltagecharacteristics without the fan connected to the power supply, whereinnothing was connected to the power supply except the measurement device.This arrangement is referred to as NO LOAD.

FIG. 8 shows the power supply voltage versus time for the cases of FAN,NO FILTER; RC FILTER; RQF NO INDUCTOR; RQF WITH INDUCTOR; NESTED RQF. Inthis figure, significant noise is visible for FAN, NO FILTER. Not onlyis approximately 35 mV peak-to-peak (mVpp) continuous ripple observed,but large voltage spikes of up to 500 mV peak (mVp) are observed. Somesmall ripple is visible for RC FILTER. For the remaining traces, nosignificant ripple is obvious in FIG. 8.

FIG. 9 shows the same traces of FIG. 8, excluding the first trace: FAN,NO FILTER. The scale is magnified by a factor of ten, rendering theremaining ripple on the traces more visible and more easily comparable.On this figure the RC FILTER ripple is 7.79 mVpp over the data taken forthis figure; RQF NO INDUCTOR shows a significant decrease from thisvalue, being 1.19 mVpp; RQF WITH INDUCTOR shows a decrease in turn,being 0.47 mVpp; NESTED RQF shows the lowest voltage fluctuations at thepower supply of 0.28 mVpp.

FIG. 10 shows the voltage spectral density at the power supply forvarious arrangements and filter embodiments. These measurements indicatethe voltage noise present in each 1-Hz frequency bin from approximately30 Hz out to 100 kHz, the unit on the vertical scale of mVrms/sqrt(Hz)being typical of this type of measurement. Trace A) is NO LOAD, and isessentially the noise floor of the measurement series, recorded with thepower supply connected to the measurement instrument, in the absence ofload or filter. Trace B) is FAN, NO FILTER, showing considerable noisecontent across the whole given frequency range. Each subsequent traceshows progressive noise reduction; the order of decreasing voltagespectral density being C) RC FILTER, D) RQF (this embodiment includesinductor 260), and finally E) NESTED RQF. The results from this finalembodiment approach the measurement floor from approximately 40 kHz outto 100 kHz.

The present invention has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the invention. As such,references herein to specific embodiments and details thereof are notintended to limit the scope of the claims appended hereto. It will beapparent to those skilled in the art that modifications can be made inthe embodiments chosen for illustration without departing from thespirit and scope of the invention.

We claim:
 1. A transistor based filter for inhibiting load noise from entering a power supply, comprising: a. a first transistor with an emitter directly coupled to a power supply, a collector coupled to a load, and a base; b. a first capacitor coupled between the base and a ground terminal; and c. a first impedance coupled between the base and a node between the collector and the load.
 2. The transistor-based filter of claim 1 wherein the first impedance is a resistor.
 3. The transistor-based filter of claim 1 wherein the first impedance is an inductor.
 4. The transistor-based filter of claim 1 wherein the first impedance is a combination of a resistor and an inductor.
 5. The transistor-based filter of claim 1 further comprising at least one diode coupled between the base and the collector of the first transistor, wherein an anode of the at least one diode is coupled to the base and a cathode of the at least one diode is coupled to the collector, wherein the diodes are coupled in series.
 6. The transistor-based filter of claim 1 further comprising at least one zener diode coupled between the base and the collector of the first transistor, wherein a cathode of the at least one zener diode is coupled to the base and an anode of the at least one zener diode is coupled to the collector, wherein the zener diodes are coupled in series.
 7. The transistor-based filter of claim 1 further comprising diodes and zener diodes between the base and the collector of the first transistor, wherein the diodes and zener diodes are coupled in series.
 8. The transistor-based filter of claim 1 further comprising a first resistor inserted between the first impedance and the node between the first impedance and first resistor to the ground terminal.
 9. The transistor-based filter of claim 1 wherein the load is a fan or a pump. 